The present invention relates to methods of separating plastics and to plastic container recycling processes.
Plastic beverage bottles including gas transport resistant walls formed from polyethylene terepthalate or "PET" have supplanted other containers for carbonated beverages in recent years, particularly in 1-liter and larger sizes. The widespread use of these containers has created a significant need for methods of recycling the used containers. Many PET-walled beverage containers include base caps or reinforcements formed from relatively low cost polyolefin plastics such as high density polyethylene. Used containers recovered from the municipal waste stream, as by a community recycling effort or the like, may also include polyolefin-walled bottles such as common polyethylene milk containers together with the PET-walled bottles. To provide salable recycled products, the polyolefin and PET components must be separated from one another. Moreover, used containers as received at the recycling plant ordinarily include aluminum caps and contaminants such as paper labels, adhesives holding the labels onto the containers and dirt such as beverage residues. The recycling process should be capable of segregating these contaminants from the PET and polyolefins.
The broad outlines of a recycling process for used bottles of this nature have long been known to those skilled in the plastics recycling art. This known process begins with the comminution of the incoming bottles and air classification of the resulting fragments to remove loose labels, dirt and fine particles. Following classification, the remaining fragments are washed in a hot detergent solution to remove the remaining paper label fragments and adhesives. The detergent solution is then drained from the fragments and any residual detergent solution is thoroughly rinsed from the drained chips with water. After rinsing, the fragments are separated by flotation in water. As PET and aluminum are denser than water, whereas polyolefins are less dense than water, the PET and aluminum fragments sink, and the polyolefin fragments float. After drying, the floating or lighter fragments consist essentially of pure polyolefin. The PET and aluminum in the heavier fragments can be separated from one another after drying by electrostatic or other means. These segregated fragments provide separate streams of substantially pure PET and aluminum.
Although this general process outline has been widely regarded as a promising approach to the problem of plastic bottle recycling, there have been significant unmet needs heretofore for improvements in the individual steps and in the process as a whole. Considerable practical difficulties have been encountered heretofore in the flotation step of the process. It has been difficult to obtain complete separation of the PET and polyolefin fractions in the product stream. Typically, each stream has been contaminated by a considerable portion of the other plastic. This phenomenon is commonly referred to as "cross-contamination". These difficulties become more severe as attempts are made to increase the throughput rate of the process. Plastic containers typically have very thin walls. When these containers are comminuted, the resulting fragments are in the form of thin, platelike flakes having one very small dimension and two parallel oppositely-facing planar or gently curved surfaces. Despite the substantial difference in density between PET and polyolefin flakes, flotation separation results obtained with these flakes heretofore have been unacceptable.
The rinsing operation has also involved considerable practical difficulties. Large amounts of water have been consumed in thoroughly rinsing the detergent from the flakes prior to the separation step. As the rinse water ordinarily is not recycled or is only partially recycled, rinse water consumption affects the amount of fresh water required by the process, and also affects the amount of used water discharged to municipal sewers or to the environment. Reduction in the time and equipment requirements for the rinsing step of the process would improve the economics of the process.
Accordingly, there have been substantial needs heretofore for improvements in the recycling process and in the operations employed therein.